The present invention relates to methods and apparatuses for controlling multistage systems, and particularly to methods and apparatuses for controlling multistage systems in HVACandR applications.
A multistage system comprises a plurality of subsystems with each subsystem contributing something to overall performance. Examples of multistage systems found in the heating, ventilating, air-conditioning, and refrigeration (HVACandR) industry are: commercial refrigeration units used for food storage, rooftop DX cooling systems, and building chiller systems. Typical multi-stage systems can be divided into two types: systems which include one or more stage capable of being modulated or operated in an analog fashion, e.g., by using variable speed drives (VSD) on compressors, and digital systems wherein all stages are limited to either being on or off. In both types of systems, the stages may have equal or unequal capacities.
Typical control systems for controlling HVACandR systems receive a command from a user selected input at, for example, a thermometer, and feedback from inside a room or compartment under control indicating the actual temperature therein. Based on this input, a controller activates a number of stages or devices to move from the actual temperature to the selected temperature.
In the simplest case, where all stages are of the same capacity, a prior art controller would activate or deactivate new stages one-by-one as the load increases or decreases. In situations where each stage or device has a different capacity, improved control resolution can be obtained by devising a table of different stage combinations. Each stage combination would represent various digital on/off states for individual devices, where application of each stage combination would produce a particular total output capacity from the multistage system. The table would contain these various combinations ordered according to deliverable capacity. In prior art methods, stage combination tables are created manually and the controller changes between combinations by working sequentially up or down the table depending on whether load is increasing or decreasing. Stage combination tables are established based on the number of stages in a system and the capacity of each stage, and are therefore specific to a selected system. The stage combination tables are stored in the memory of the controller for retrieval during operation of the control device. In applications that include stages that can be modulated to provide intermediate capacity levels, information related to the modulation capabilities of these stages can also be stored.
To avoid instability at transition points between stage combinations, prior art controllers typically employ timer delays and/or deadbands. Timer delays and/or deadbands are employed to verify that a change in an input signal corresponds to an actual requested change, and therefore to filter out changes due to noise or other external factors which may have caused a temporary change in the measured signal.
In an all-digital system, a deadband is defined around setpoint and a change in a stage combination is invoked only when the measured signal is maintained outside of the deadband for a sustained period, as determined by the delay timers. A different combination of stages corresponding to a higher or lower capacity is activated according to whether the deadband is exceeded at its upper or lower limit, moving sequentially through all intermediate stage as noted above. In an all-digital system, load points that are between defined stage combinations can only be reached by quickly moving between the straddling stage combinations.
In a system including individual stages capable of being modulated such as an air-handling unit with sequenced heating, cooling, and heat-recovery, a feedback controller can be used to modulate the output of one particular stage until the controller saturates high or low (see xe2x80x9cA New Sequencing Control Strategy for Air-andling Unitsxe2x80x9d, Seem, J. E. et al., International Journal of Heating, Ventilating, Air-conditioning, and Refrigeration Research, Volume 5, Number 1, January 1999, pp. 35-38). A time delay is then applied so that a new stage combination is only invoked when the controller has been saturated for a sustained period. When a new combination is activated, the feedback controller switches so that it controls another device leaving the previous device in its saturated state, either fully on or fully off. Sometimes, a deadband is also incorporated so that a change in stage combination is only made when the setpoint error is large enough to exceed the deadband. This prevents changes in stage combinations when the controller has saturated but there is little or no demand for capacity change. As in the digital control method described above, control performance is determined by the time delay and the magnitude of any deadbands that are used.
While the typical prior art systems described above are generally useful in controlling a multistage system, there are significant problems associated with each of these systems. First, as noted above, typical control systems require the establishment of manual stage combination tables, which are generated for a specific application. The associated controller is therefore tied to a specific application, and changes to the controller are generally required when changes are made to the controlled stages, when new stages are added, or when stages are removed. Secondly, the deadzone and time delay methods employed in typical prior art systems produce significant time delays and sluggish control response, particularly when significant changes. in capacity are requested. Faced with a large change in demand, a conventional controller designed around deadbands and timers must wait for the designated delay time at each intermediate stage combination before being able to move to the next. For large disturbances, such as start-up transients, there would therefore be a compounded delay time that would make the control sluggish. These problems are compounded by the fact that prior art controllers typically step sequentially through all stages, delaying at each one before reaching at suitable stage combination corresponding to the requested setpoint. Additionally, prior art controllers also require selection of a transition time delay, which is difficult to relate to any measure of control performance. Having a fixed time delay also means that the control methods will be equally as sensitive to large and small setpoint errors. This is counter-intuitive as a more rapid response and hence small delay is desirable for large errors, while a longer delay may be tolerable for smaller errors.
U.S. Pat. No. 5,440,891 to Hindmon proposes an alternative prior art controller based on a fuzzy logic algorithm. Here, the fuzzy logic controller determines an appropriate combination of stages to activate based on the measured controlled variable. The controller is capable of moving to a new stage combination without having to pass through intermediate stages. The delay in moving between different stage combinations is also variable allowing rapid reaction to large disturbances. While responding to certain deficiencies in the prior art, however, there are also problems associated with the fuzzy logic approach. Specifically, in a fuzzy logic system, the performance of the controller is determined by a predetermined set of fuzzy rules and other internal parameters, and is therefore specifically tuned for a given application. Variable levels of control performance are obtained when the method is applied to different systems. Re-configuration or tuning for a specific application can be difficult, time consuming, and inefficient, and could require an entire recreation of the fuzzy rule set.
The present invention is a control method and apparatus for use with or in multistage systems that is particularly suited for use in the HVACandR industry, such as in the aforementioned examples of food storage refrigeration systems, rooftop DX cooling units, and building chiller systems.
The control method of the present invention combines a table of stage combinations with a hysteretic deadzone and a split range control method to produce a new control method that can be applied to a general multistage control system where the capacities (gains) of the stages are known a priori. The present invention further automates the generation of stage combination tables to provide a flexible control system which can be easily modified. Unlike, prior art methods the present invention can produce consistent control performance across classes of similar systems without the need to tune each particular implementation.
In one aspect of the invention, combinations of stage states (hereafter referred to as xe2x80x9cstage combinationsxe2x80x9d) in which selected stages are either fully on or fully off are automatically generated and ordered according to deliverable capacity. At least one stage in each stage combination is left inactive such that this stage can be individually controlled by means of pulsing or modulation to provide contiguous control between the discrete outputs available (in steady-state) at the different on/off stage combinations. Data necessary for constructing stage combinations including the number, relative capacity, and type of stages in the system are provided by a user, and are therefore individualized for each system to which a controller constructed in accordance with the present invention is used. The automatic construction of stage combinations obviates the need for manual tables of stage combinations, and allows a controller constructed in accordance with the present invention to be used in conjunction with or moved between different multistage systems.
In another aspect of the invention, stage combination tables are employed to provide a selected capacity output based on a main control signal, which provides an operational setpoint. The main control signal is used to determine a minimum capacity of the stage combination closest to, but greater than or less than the selected setpoint, depending on whether the command is increasing or decreasing. The main control signal is also used to calculate a xe2x80x9csplit rangexe2x80x9d signal corresponding to the amount of capacity that must be provided by the individually-controlled stage in the stage combination. Hysteretic deadzones are centered on the points where transitions occur between stage combinations. The deadzones define a region around each stage combination transition point in which a change in stage combination is not allowed. In these deadzone regions, the split range control is saturated to maintain the individually controlled stage at its minimum or maximum value, depending on whether demand is decreasing (i.e., moving to a lower capacity stage) or increasing (moving to a higher capacity stage), and is maintained in this state until the main control signal exceeds the deadzone. After the deadzone is exceeded, the next stage combination to be activated is selected based on the magnitude of the main control signal. The control does not xe2x80x9cstep throughxe2x80x9d or automatically switch to the next higher or lower capacity stage combination as is typical in prior art devices, but can select any available stage combinations depending on the magnitude of the setpoint.
The method and apparatus of the present invention therefore provides a number of notable advantages over typical prior art devices. First, in the present invention, stage combination tables that provide for a contiguous control range can be automatically generated and revised when new or different stages are added to a system. These systems can be easily controlled by a network or other communications system, and changes in the multistage system can be easily implemented without changes to hardware configurations. Additionally, through the use of hysteretic deadzones in combination with a split range control method, the control system of the present invention reduces instability new stage transition points. Furthermore, the control method,of the present invention provides a relatively quick response time to disturbances, and particularly to large disturbances in setpoint, since the present invention allows for a xe2x80x9cjumpxe2x80x9d to a non-sequential stage in the stage combination table, thereby allowing for significant changes in output capacity relatively quickly.
These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention.